Gerd Keiser - Optical Fiber Communications-McGraw-Hill Education (2010).pdf

Optical Fiber Communications
——Fourth Edition——

Optical Fiber Communications
Fourth Edition
Gerd Keiser
National Taiwan University of Science and Technology

OPTICAL FIBER COMMUNICATIONS, FOURTH EDITION
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Library of Congress Cataloging-in-Publication Data
Keiser, Gerd.
Optical ﬁber communications/Gerd Keiser.—4th ed.
p. c
m.
ISBN 978-0-07-338071-1 (alk. paper)
1. Optical c
ommunications. 2. Fiber opt
ics. I. Title.
TK5103.59.K44 2011
621.382’75—dc22
2010023150
www.mhhe.com

About the Author
Gerd Keiser is a National Science Council Chair Professor in the Department of Electronic Engineering
at the National Taiwan University of Science and Technology. His teaching and research interests include
photonic component development, telecom optical transmission systems, ﬁber-to-the-premises (FTTP)
networks, intelligent-building and smart-home networks, and biomedical photonics. In addition, he is
the founder and principal consultant at PhotonicsComm Solutions, a ﬁrm specializing in consulting and
education for the optical communications industry. Previously he worked at Honeywell, GTE, and General
Dynamics in the development and application of optical network and digital switch technologies. His
technical achievements at GTE earned him the prestigious Leslie WarnerAward.As an outside activity, he
was an adjunct professor at Northeastern University, Tufts University, and Boston University. Dr. Keiser
is a Fellow of the IEEE, a member of OSA and SPIE, an associate editor of the journal Optical Fiber
Technology, and the author of four graduate-level books.

Brief Contents
Chapter 1 Overview of Optical Fiber Communications
Chapter 2 Optical Fibers: Structures, Waveguiding, and Fabrication
Chapter 3 Attenuation and Dispersion
Chapter 4 Optical Sources
Chapter 5 Power Launching and Coupling
Chapter 6 Photodetectors
Chapter 7 Optical Receiver Operation
Chapter 8 Digital Links
Chapter 9 Analog Links
Chapter 10 WDM Concepts and Components
Chapter 11 Optical Ampliﬁers
Chapter 12 Nonlinear Effects
Chapter 13 Optical Networks
Chapter 14 Performance Measurement and Monitoring
Appendixes
Index

Contents
Preface xxii
Acknowledgments xxvii
1 Overview of Optical Fiber Communications 1
1.1 Motivations for Lightwave Communications 2
1.1.1 The Path to Optical Networks 2
1.1.2 Advantages of Optical Fibers 5
1.2 Optical Spectral Bands 5
1.2.1 Electromagnetic Energy 5
1.2.2 Windows and Spectral Bands 8
1.3 Decibel Units 9
1.4 Network Information Rates 12
1.4.1 Telecom Signal Multiplexing 12
1.4.2 SONET/SDH Multiplexing Hierarchy 13
1.5 WDM Concepts 15
1.6 Key Elements of Optical Fiber Systems 15
1.7 Standards for Optical Fiber Communications 19
1.8 Modeling and Simulation Tools 20
1.8.1 Simulation Tool Characteristics 20
1.8.2 Graphical Programming 21
1.8.3 Example Programs for Student Use 23
Problems 23
References 24
2 Optical Fibers: Structures, Waveguiding, and Fabrication 27
2.1 The Nature of Light 27
2.1.1 Linear Polarization 28
2.1.2 Elliptical and Circular Polarization 31
2.1.3 The Quantum Nature of Light 33
2.2 Basic Optical Laws and Deﬁnitions 33
2.2.1 Refractive Index 33
2.2.2 Reﬂection and Refraction 34

x Contents
2.2.3 Polarization Components of Light 36
2.2.4 Polarization-Sensitive Materials 38
2.3 Optical Fiber Modes and Conﬁgurations 39
2.3.1 Fiber Types 40
2.3.2 Rays and Modes 42
2.3.3 Step-Index Fiber Structure 43
2.3.4 Ray Optics Representation 43
2.3.5 Wave Representation in a Dielectric Slab Waveguide 45
2.4 Mode Theory for Circular Waveguides 47
2.4.1 Overview of Modes 48
2.4.2 Summary of Key Modal Concepts 49
2.4.3 Maxwell’s Equations* 51
2.4.4 Waveguide Equations* 52
2.4.5 Wave Equations for Step-Index Fibers* 54
2.4.6 Modal Equation* 55
2.4.7 Modes in Step-Index Fibers* 57
2.4.8 Linearly Polarized Modes* 60
2.4.9 Power Flow in Step-Index Fibers* 63
2.5 Single-Mode Fibers 65
2.5.1 Construction 65
2.5.2 Mode-Field Diameter 65
2.5.3 Propagation Modes in Single-Mode Fibers 67
2.6 Graded-Index Fiber Structure 68
2.7 Fiber Materials 70
2.7.1 Glass Fibers 70
2.7.2 Active Glass Fibers 71
2.7.3 Plastic Optical Fibers 71
2.8 Photonic Crystal Fibers 72
2.8.1 Index-Guiding PCF 72
2.8.2 Photonic Bandgap Fiber 73
2.9 Fiber Fabrication 74
2.9.1 Outside Vapor-Phase Oxidation 75
2.9.2 Vapor-Phase Axial Deposition 75
2.9.3 Modiﬁed Chemical Vapor Deposition 76
2.9.4 Plasma-Activated Chemical Vapor Deposition 76
2.9.5 Photonic Crystal Fiber Fabrication 77

Contents xi
2.10 Mechanical Properties of Fibers 78
2.11 Fiber Optic Cables 82
2.11.1 Cable Structures 83
2.11.2 Indoor Cable Designs 84
2.11.3 Outdoor Cables 86
2.12 Cable Installation Methods 87
2.12.1 Direct-Burial Installations 87
2.12.2 Pulling Cable into Ducts 88
2.12.3 Cable Jetting Installation 89
2.12.4 Aerial Installation 90
2.12.5 Underwater Installation 91
2.12.6 Industry Installation Standards 92
Problems 93
References 96
3 Attenuation and Dispersion 102
3.1 Attenuation 102
3.1.1 Attenuation Units 103
3.1.2 Absorption 104
3.1.3 Scattering Losses 108
3.1.4 Bending Losses 111
3.1.5 Core and Cladding Losses 113
3.2 Signal Dispersion in Fibers 114
3.2.1 Overview of Dispersion Origins 114
3.2.2 Modal Delay 116
3.2.3 Factors Contributing to Dispersion 118
3.2.4 Group Delay 119
3.2.5 Material Dispersion 121
3.2.6 Waveguide Dispersion 123
3.2.7 Dispersion in Single-Mode Fibers 124
3.2.8 Polarization-Mode Dispersion 126
3.3 Characteristics of Single-Mode Fibers 128
3.3.1 Refractive-Index Proﬁles 128
3.3.2 Cutoff Wavelength 130
3.3.3 Dispersion Calculations 132
3.3.4 Mode-Field Diameter 135
3.3.5 Bending Loss 135

xii Contents
3.4 International Standards 138
3.4.1 Recommendation G.651.1 138
3.4.2 Recommendation G.652 138
3.5 Specialty Fibers 142
Problems 145
References 147
4 Optical Sources 151
4.1 Topics from Semiconductor Physics 152
4.1.1 Energy Bands 152
4.1.2 Intrinsic and Extrinsic Material 154
4.1.3 The pn Junctions 156
4.1.4 Direct and Indirect Bandgaps 157
4.1.5 Semiconductor Device Fabrication 158
4.2 Light-Emitting Diodes (LEDs) 158
4.2.1 LED Structures 158
4.2.2 Light Source Materials 161
4.2.3 Quantum Efﬁciency and LED Power 165
4.2.4 Modulation of an LED 168
4.3 Laser Diodes 170
4.3.1 Laser Diode Modes and Threshold Conditions 171
4.3.2 Laser Diode Rate Equations 177
4.3.3 External Quantum Efﬁciency 178
4.3.4 Resonant Frequencies 179
4.3.5 Laser Diode Structures and Radiation Patterns 180
4.3.6 Single-Mode Lasers 185
4.3.7 Modulation of Laser Diodes 189
4.3.8 Laser Linewidth 190
4.3.9 External Modulation 191
4.3.10 Temperature Effects 193

Contents xiii
4.4 Line Coding 195
4.4.1 NRZ and RZ Signal Formats 195
4.4.2 Block Codes 196
4.5 Light Source Linearity 196
4.6 Reliability Considerations 198
4.7 Transmitter Packages 202
Problems 204
References 208
5 Power Launching and Coupling 211
5.1 Source-to-Fiber Power Launching 212
5.1.1 Source Output Pattern 212
5.1.2 Power-Coupling Calculation 213
5.1.3 Power Launching versus Wavelength 216
5.1.4 Equilibrium Numerical Aperture 217
5.2 Lensing Schemes for Coupling Improvement 218
5.2.1 Nonimaging Microsphere 219
5.2.2 Laser Diode-to-Fiber Coupling 220
5.3 Fiber-to-Fiber Joints 221
5.3.1 Mechanical Misalignment 223
5.3.2 Fiber-Related Losses 229
5.3.3 Fiber End-Face Preparation 230
5.4 LED Coupling to Single-Mode Fibers 231
5.5 Fiber Splicing 232
5.5.1 Splicing Techniques 233
5.5.2 Splicing Single-Mode Fibers 234
5.6 Optical Fiber Connectors 235
5.6.1 Connector Types 235
5.6.2 Single-Mode Fiber Connectors 239
5.6.3 Connector Return Loss 240
Problems 242
References 244
6 Photodetectors 248
6.1 Physical Principles of Photodiodes 249
6.1.1 The pin Photodetector 249
6.1.2 Avalanche Photodiodes 253

xiv Contents
6.2 Photodetector Noise 257
6.2.1 Noise Sources 257
6.2.2 Signal-to-Noise Ratio 261
6.2.3 Noise-Equivalent Power 262
6.3 Detector Response Time 263
6.3.1 Depletion Layer Photocurrent 263
6.3.2 Response Time 264
6.3.3 Double-Heterostructure Photodiodes 267
6.4 Avalanche Multiplication Noise 267
6.5 Structures for InGaAs APDs 270
6.6 Temperature Effect on Avalanche Gain 270
6.7 Comparisons of Photodetectors 272
Problems 273
References 275
7 Optical Receiver Operation 277
7.1 Fundamental Receiver Operation 278
7.1.1 Digital Signal Transmission 278
7.1.2 Error Sources 280
7.1.3 Front-End Ampliﬁer 282
7.2 Digital Receiver Performance 283
7.2.1 Probability of Error 283
7.2.2 Receiver Sensitivity 287
7.2.3 The Quantum Limit 290
7.3 Eye Diagrams 291
7.3.1 Eye Pattern Features 291
7.3.2 BER and Q-Factor Measurements 293
7.4 Burst-Mode Receivers 294
7.5 Analog Receivers 297
Problems 300
References 302
8 Digital Links 304
8.1 Point-to-Point Links 305
8.1.1 System Considerations 306
8.1.2 Link Power Budget 307
8.1.3 Rise-Time Budget 310

Contents xv
8.1.4 Short-Wavelength Band 313
8.1.5 Attenuation-Limited Distances for Single-Mode Links 314
8.2 Power Penalties 316
8.2.1 Chromatic Dispersion Penalty 316
8.2.2 Polarization-Mode Dispersion Penalty 318
8.2.3 Extinction Ratio Penalty 319
8.2.4 Modal Noise 319
8.2.5 Mode-Partition Noise 322
8.2.6 Chirping 323
8.2.7 Reflection Noise 325
8.3 Error Control 327
8.3.1 Concept of Error Detection 328
8.3.2 Linear Error Detection Codes 328
8.3.3 Polynomial Codes 329
8.3.4 Forward Error Correction 331
8.4 Coherent Detection 332
8.4.1 Fundamental Concepts 333
8.4.2 Homodyne Detection 335
8.4.3 Heterodyne Detection 335
8.4.4 BER Comparisons 336
8.5 Differential Quadrature Phase-Shift Keying 342
Problems 343
References 345
9 Analog Links 350
9.1 Overview of Analog Links 351
9.2 Carrier-to-Noise Ratio 352
9.2.1 Carrier Power 352
9.2.2 Photodetector and Preamplifier Noises 353
9.2.3 Relative Intensity Noise (RIN) 354
9.2.4 Reflection Effects on RIN 354
9.2.5 Limiting Conditions 356
9.3 Multichannel Transmission Techniques 357
9.3.1 Multichannel Amplitude Modulation 358
9.3.2 Multichannel Frequency Modulation 361
9.3.3 Subcarrier Multiplexing 362

xvi Contents
9.4 RF over Fiber 364
9.4.1 Key Link Parameters 364
9.4.2 Spur-Free Dynamic Range 366
9.5 Radio-over-Fiber Links 368
9.5.1 Antenna Base Station ROF Network 368
9.5.2 Radio-over-Multimode Fiber 369
9.6 Microwave Photonics 370
Problems 371
References 373
10 WDM Concepts and Components 376
10.1 Overview of WDM 377
10.1.1 Operational Principles of WDM 377
10.1.2 WDM Standards 379
10.2 Passive Optical Couplers 381
10.2.1 The 2 ¥ 2 Fiber Coupler 381
10.2.2 Scattering Matrix Representation 385
10.2.3 The 2 ¥ 2 Waveguide Coupler 387
10.2.4 Star Couplers 391
10.2.5 Mach-Zehnder Interferometer Multiplexers 393
10.3 Isolators and Circulators 396
10.3.1 Optical Isolators 396
10.3.2 Optical Circulators 398
10.4 Fiber Grating Filters 399
10.4.1 Grating Basics 399
10.4.2 Fiber Bragg Grating 399
10.4.3 FBG Applications 402
10.5 Dielectric Thin-Film Filters 403
10.5.1 Etalon Theory 404
10.5.2 TFF Applications 406
10.6 Phased-Array-Based Devices 407
10.7 Diffraction Gratings 412
10.8 Active Optical Components 413
10.8.1 MEMS Technology 413
10.8.2 Variable Optical Attenuators 414
10.8.3 Tunable Optical Filters 415

Contents xvii
10.8.4 Dynamic Gain Equalizers 416
10.8.5 Optical Add/Drop Multiplexers 417
10.8.6 Polarization Controllers 418
10.8.7 Chromatic Dispersion Compensators 418
10.9 Tunable Light Sources 419
Problems 421
References 425
11 Optical Amplifiers 429
11.1 Basic Applications and Types of Optical Amplifiers 429
11.1.1 General Applications 430
11.1.2 Amplifier Types 431
11.2 Semiconductor Optical Amplifiers 433
11.2.1 External Pumping 433
11.2.2 Amplifier Gain 435
11.2.3 SOA Bandwidth 436
11.3 Erbium-Doped Fiber Amplifiers 437
11.3.1 Amplification Mechanism 437
11.3.2 EDFA Architecture 439
11.3.3 EDFA Power-Conversion Efficiency and Gain 441
11.4 Amplifier Noise 445
11.5 Optical SNR 449
11.6 System Applications 450
11.6.1 Power Amplifiers 451
11.6.2 In-Line Amplifiers 451
11.6.3 Preamplifiers 453
11.6.4 Multichannel Operation 453
11.6.5 In-Line Amplifier Gain Control 455
11.7 Raman Amplifiers 456
11.8 Wideband Optical Amplifiers 460
Problems 461
References 464
12 Nonlinear Effects 468
12.1 General Overview of Nonlinearities 469
12.2 Effective Length and Area 469

xviii Contents
12.3 Stimulated Raman Scattering 471
12.4 Stimulated Brillouin Scattering 473
12.5 Self-Phase Modulation 475
12.6 Cross-Phase Modulation 476
12.7 Four-Wave Mixing 477
12.8 FWM Mitigation 479
12.9 Wavelength Converters 480
12.9.1 Optical-Gating Wavelength Converters 480
12.9.2 Wave-Mixing Wavelength Converters 481
12.10 Solitons 482
12.10.1 Soliton Pulses 483
12.10.2 Soliton Parameters 485
12.10.3 Soliton Width and Spacing 487
Problems 489
References 490
13 Optical Networks 493
13.1 Network Concepts 494
13.1.1 Network Terminology 494
13.1.2 Network Categories 495
13.1.3 Network Layers 497
13.1.4 Optical Layer 499
13.2 Network Topologies 499
13.2.1 Performance of Passive Linear Buses 501
13.2.2 Performance of Star Architectures 506
13.3 SONET/SDH 507
13.3.1 Transmission Formats and Speeds 507
13.3.2 Optical Interfaces 509
13.3.3 SONET/SDH Rings 511
13.3.4 SONET/SDH Networks 515
13.4 High-Speed Lightwave Links 516
13.4.1 Links Operating at 10 Gb/s 517
13.4.2 Links Operating at 40 Gb/s 519
13.4.3 Standards for 40-Gigabit and 100-Gigabit Ethernet 519
13.4.4 OTDM Links Operating at 160 Gb/s 520

Contents xix
13.5 Optical Add/Drop Multiplexing 521
13.5.1 OADM Configurations 522
13.5.2 Reconfigurable OADM 523
13.6 Optical Switching 528
13.6.1 Optical Crossconnect 529
13.6.2 Wavelength Conversion 530
13.6.3 Wavelength Routing 533
13.6.4 Optical Packet Switching 534
13.6.5 Optical Burst Switching 535
13.7 WDM Network Examples 537
13.7.1 Wideband Long-Haul WDM Networks 537
13.7.2 Narrowband Metro WDM Networks 539
13.8 Passive Optical Networks 540
13.8.1 Basic PON Architectures 540
13.8.2 Active PON Modules 542
13.8.3 Traffic Flows 544
13.8.4 GPON Characteristics 544
13.8.5 WDM PON Architectures 548
13.9 IP over DWDM 548
13.10 Optical Ethernet 550
13.10.1 Basic Optical Ethernet Implementations 550
13.10.2 EPON/GE-PON Architecture 552
13.10.3 Metro Optical Ethernet 553
13.11 Mitigation of Transmission Impairments 553
13.11.1 Chromatic Dispersion Compensating Fiber 554
13.11.2 Bragg Grating Dispersion Compensators 555
13.11.3 Polarization-Mode Dispersion Compensation 556
13.11.4 Optical Amplifier Gain Transients 557
Problems 558
References 563
14 Performance Measurement and Monitoring 570
14.1 Measurement Standards 571
14.2 Basic Test Equipment 573
14.2.1 Test-Support Lasers 574
14.2.2 Optical Spectrum Analyzer 575

xx Contents
14.2.3 Multiple-Function Testers 576
14.2.4 Optical Power Attenuators 576
14.2.5 Optical Transport Network (OTN) Tester 576
14.2.6 Visual Fault Indicator 577
14.3 Optical Power Measurements 577
14.3.1 Deﬁnition of Optical Power 577
14.3.2 Optical Power Meters 578
14.4 Optical Fiber Characterization 578
14.4.1 Refracted Near-Field Technique 579
14.4.2 Transmitted Near-Field Technique 579
14.4.3 Attenuation Measurements 579
14.4.4 Dispersion Measurements 582
14.5 Eye Diagram Tests 588
14.5.1 Mask Testing 589
14.5.2 Stressed Eye 590
14.5.3 Eye Contour 590
14.6 Optical Time-Domain Reflectometer 591
14.6.1 OTDR Trace 592
14.6.2 Attenuation Measurements 593
14.6.3 OTDR Dead Zone 594
14.6.4 Fiber Fault Location 595
14.6.5 Optical Return Loss 595
14.7 Optical Performance Monitoring 596
14.7.1 Management Architecture and Functions 597
14.7.2 Optical Layer Management 598
14.7.3 OPM Functions 600
14.7.4 Network Maintenance 601
14.7.5 Fault Management 602
14.7.6 OSNR Monitoring 603
14.8 Optical Fiber System Performance Measurements 603
14.8.1 Bit-Error Rate Testing 604
14.8.2 Optical Signal-to-Noise Ratio (OSNR) Estimation 605
14.8.3 Q Factor Estimation 607

Contents xxi
14.8.4 Optical Modulation Amplitude (OMA) Measurement 609
14.8.5 Timing Jitter Measurement 610
Problems 611
References 613
Appendix A International System of Units 617
Appendix B Useful Mathematical Relations 618
Appendix C Bessel Functions 622
Appendix D Decibels 625
Appendix E Acronyms 627
Appendix F Roman Symbols 633
Appendix G Greek Symbols 636
Index 639

Preface
Objective
Optical fiber communications has undergone a fascinating history since the first edition of this book in 1983.
Especially exciting was the 2009 Nobel Prize in Physics received by Charles K. C. Kao for his pioneer-
ing insight into using glass fibers as a medium for data transmission and for his enthusiastic international
follow-ups in promoting the further development of low-loss optical fibers. The first ultrapure fiber was
fabricated in 1970, only four years after Kao’s prediction. This breakthrough led to a series of technology
developments related to optical fibers. Initially the technology focused on simple transmission links but then
quickly moved to increasingly sophisticated networks.Along the way many new components and commu-
nication techniques were tried. Some of these were highly successful, some faded away perhaps because of
their implementation complexity, and others, which were ahead of their time, are reappearing after being in
hibernation for a while. Modern high-capacity telecommunication networks based on optical fiber technol-
ogy now have become an integral and indispensable part of society. Applications for these sophisticated
networks range from simple web browsing and e-mail exchanges to critical health care diagnosis, grid and
cloud computing, and complex business transactions. Due to the importance of these networks to everyday
life, users have come to expect the communication services to always be available and to function properly.
Meeting such a stringent demand requires careful engineering in all technological aspects ranging from
component development to system design and installation to network operation and maintenance.
To address the attainment and implementation of these skills, this expanded fourth edition presents
the fundamental principles for understanding and applying a wide range of optical fiber technologies to
modern communication networks. The sequence of topics takes the reader systematically from the under-
lying principles of components and their interactions with other devices in an optical fiber link, through
descriptions of the architectures and performance characteristics of complex optical links and networks,
to essential measurement and test procedures required during network installation and maintenance. By
mastering these fundamental topics the reader will be prepared not only to contribute to disciplines such
as current device, communication link, or equipment designs, but also to understand quickly any further
technology developments for future enhanced networks.
Contents
To accomplish these objectives, Chapter 1 gives a basic overview of how optical communications blend
into telecommunication systems. The discussions include the motivations and advantages for using optical
fibers, the spectral bands being used, how wavelength division multiplexing can boost the transmission
capacity of an optical fiber, what standards are being applied, and the use of simulation tools.
Chapters 2 through 11 describe the purpose and performance characteristics of the major elements in an
optical link. These elements include optical fibers, light sources, photodetectors, passive optical devices,
optical amplifiers, and active optoelectronic devices used in multiple-wavelength networks. Despite its
apparent simplicity, an optical fiber is one of the most important elements in a fiber link. Chapters 2 and 3

Preface xxiii
give details on the physical structures, constituent materials, attenuation behavior, lightwave propagation
mechanisms, and signal distortion characteristics of the wide variety of optical fibers that exist. In ad-
dition, Chapter 3 introduces optical fiber fabrication methods and illustrates several generic fiber cable
configurations. The new topics in these chapters include a discussion of photonic crystal fibers and a
streamlining of the discussions on modal effects and pulse broadening.
Chapter 4 addresses the structures, light-emitting principles, and operating characteristics of light
sources used in optical communications. In addition, the discussion includes direct and external modulation
techniques, temperature effects, device lifetime considerations, and line coding methods for transporting
signals over a fiber. How to couple the light source to a fiber is described in Chapter 5, as well as how
to join two fibers in order to ensure a low optical power loss at the joints.
The lightwave receiver has the task of detecting the arriving optical signal and converting it into an
electrical signal that can be processed by the receiver electronics. Chapter 6 covers the structures and
responses of photodetectors, and Chapter 7 describes the principles and functions of lightwave receiv-
ers. The new features in Chapter 7 include a simplified presentation of the operational characteristics of
lightwave receivers, the concepts of signal-detection statistics, eye-diagram measurement schemes, and
a description of burst-mode receivers used for passive optical networks.
Chapters 8 and 9 examine the design methods for digital and analog links, respectively. For Chapter 8 this
includes discussions of link power budgets and bandwidth limitations. The new features are an expanded
coverage of power penalties, basic coherent detection schemes, and details of error-control methods for
digital signals.Additions to Chapter 9 include the concepts of sending radio-frequency (RF) analog signals
at microwave frequencies over optical fibers.An expanding application of these RF-over-fiber techniques
is for broadband radio-over-fiber networks for mobile subscribers outdoors and in buildings.
Chapter 10 addresses the principles of wavelength division multiplexing (WDM), examines the func-
tions of a generic WDM link, and discusses international standards for different WDM schemes. New
features in this chapter include expanded descriptions and application examples of passive and active
WDM devices, such as fiber Bragg gratings, thin-film filters, arrayed waveguide gratings, diffraction
gratings, and variable optical attenuators.
Chapter 11 describes different concepts for creating optical amplification.Among the topics are semi-
conductor optical amplifiers, doped-fiber amplifiers, and a new section on Raman amplification schemes.
In addition to discussions of the traditional erbium-doped fiber amplifier (EDFA), new structures such as
a thulium-doped fiber amplifier (TDFA) for use in the S-band and a gain-shifted EDFA for the L-band
are described.
Next, Chapters 12 through 14 show how the elements are put together to form links and networks,
and explain measurement methodologies used to evaluate the performance of lightwave components
and links. A new Chapter 12 is devoted to the origins and effects of nonlinear processes in optical fibers.
Some of these nonlinear effects degrade system performance and need to be controlled, whereas others,
such as stimulated Raman scattering, can have beneficial uses.
Optical networking concepts for long-haul, metro, local area, and access networks are presented in
an extensively expanded Chapter 13. Among the new topics are high-speed optical links operating up
to 160 Gb/s, the concepts of optical add/drop multiplexing and optical crossconnects, wavelength rout-
ing, optical packet switching, optical burst switching, passive optical networks, IP over WDM, optical
Ethernet, and mitigation techniques for transmission impairments in high-speed networks.
The final chapter discusses performance measurement and monitoring. The topics include a discus-
sion of internationally recognized measurement standards, basic test instruments for optical fiber link
characterization, methods for characterizing optical fibers, and evaluation of link performance through

xxiv Preface
eye-pattern measurements. Particular emphasis is placed on evaluating WDM links. New features in
Chapter 14 concerning eye diagrams include the concepts of eye masks, stressed-eye tests, and bit-error
rate eye contours.Another new feature is a discussion of optical performance monitoring. This has become
an essential function in optical communication networks, particularly in relation to error monitoring,
network maintenance, and fault management.
New to This Edition
The following material is new to this edition of the book:
∑ Designation of spectral bands used in optical fiber communications
∑ Descriptions of photonic crystal fibers, which have an internal microstructure that adds another
dimension of light control within a fiber
∑ An overview of optical fiber cable-installation methods used in environments ranging from indoor
ducts to undersea links
∑ Descriptions of specialty fibers that are designed to interact with light in order to control and
manipulate optical signals
∑ A discussion of international standards used to designate the characteristics of various types of
optical fibers in order to have industry-wide compatibility between fibers
∑ An illustrative section on the characteristics and form factors of popular commercial transceiver
packages
∑ An illustrative section on the characteristics and form factors of popular commercial optical fiber
connectors
∑ A discussion on the characteristics of burst-mode optical receivers needed for passive optical
networks
∑ Expanded coverage of power penalties incurred in transmission links
∑ Expanded coverage of single-mode links operating at 10 Gb/s and higher
∑ A new section on coherent detection methods that offer higher spectral purity and greater resist-
ance to dispersion effects compared to conventional direct-detection methods
∑ A new section on digital quadrature phase-shift-keying (DQPSK) methods used for transmission
links operating at rates greater than 10 Gb/s
∑ A new section on digital error detection and correction methods including the use of polynomial
codes and forward-error-correction (FEC) techniques
∑ Anew section on radio-over-fiber technologies for wireless access networks, wireless services for
indoor environments, and connections to personal area networks in homes
∑ Expanded coverage of photonic devices used in wavelength division multiplexing (WDM)
∑ A new section on Raman optical amplification techniques and an expanded discussion on erbium-
doped fiber amplifiers (EDFA)
∑ A new chapter on the impact of nonlinear effects in optical fibers
∑ A significantly expanded chapter on optical networks that now includes high-speed lightwave
links, optical add/drop multiplexing, optical switching, WDM network examples, IP over WDM,
optical Ethernet, and passive optical networks for fiber-to-the-premises (FTTP) applications
∑ A revised chapter on performance measurement and monitoring that includes eye diagram tests,
optical performance monitoring (OPM) functions, and performance testing methods; the measure-
ment tests include bit-error rate (BER), optical signal-to-noise ratio (OSNR), the Q factor, optical
modulation amplitude (OMA), and timing jitter

Preface xxv
Use of the Book
This fourth edition provides the basic material for a senior-level or graduate course in the theory and
application of optical fiber communication technology. It also will serve well as a working reference
for practicing engineers dealing with the design and development of components, transmission equip-
ment, test instruments, and cable plants for optical fiber communication systems. The background
required to study the book is that of typical senior-level engineering students. This includes introductory
electromagnetic theory, calculus and elementary differential equations, and basic concepts of optics as
presented in a freshman physics course. Concise reviews of several background topics, such as optics
concepts, electromagnetic theory, and basic semiconductor physics, are included in the main body of
the text. Various sections dealing with advanced material (e.g., the applications of Maxwell’s equations
to cylindrical dielectric waveguides) are designated by a star and can be skipped over without loss of
continuity. To assist readers in learning the material and applying it to practical designs, 143 examples are
given throughout the book. A collection of 277 homework problems is included to help test the reader’s
comprehension of the material covered, and to extend and elucidate the text. Instructors can obtain the
problem solutions from the publisher.
Numerous references are provided at the end of each chapter as a start for delving deeper into
any given topic. Since optical fiber communications brings together research and development ef-
forts from many different scientific and engineering disciplines, there are hundreds of articles in the
literature relating to the material covered in each chapter. Even though not all these articles can be
cited in the references, the selections represent some of the major contributions to the fiber optics
field and can be considered as a good introduction to the literature. Supplementary material and
references for up-to-date developments can be found in specialized textbooks and various confer-
ence proceedings.
To help the reader understand and use the material in the book, a table in the inside front cover provides
a quick reference for various physical constants and units. Appendices A through D give an overview
of the international system of units, listings of mathematical formulas needed for homework problems,
and discussions on decibels. Appendices E through G provide listings of acronyms, Roman symbols,
and Greek symbols, respectively, that are used in the book.
Computer-based modeling and simulation tools offer a powerful method to assist in analyzing the
design of an optical component, circuit, or network before costly prototypes are built. The book website
(www.mhhe.com/keiserOFC) describes abbreviated interactive demonstration versions of simulation
modules. These modules may be downloaded from the websites of three companies that produce simula-
tion tools. The simplified versions contain over 100 predefined component and link configurations that
allow interactive concept demonstrations.Although the configurations are fixed, the user can vary certain
parameters or turn them on and off to see the effect on system performance.
Book Website
In addition to providing web links for learning about and downloading interactive simulation demon-
strations, the book website (www.mhhe.com/keiserOFC) has further information on new technology
developments and updated reference material related to the book. The problem solutions are available
to instructors, as well as a selection of lecture slides and figures from the text. These are available for
download at the password-protected section of the website for instructors only. Permission for instructors
to access the solutions manual can be obtained from the publisher.

xxvi Preface
COSMOS
(available to Instructors only)
McGraw-Hill’s COSMOS (Complete Online Solutions Manual Organization System) allows instructors to
streamline the creation of assignments, quizzes, and tests by using problems and solutions from the textbook,
as well as their own custom material. COSMOS is now available online at http://cosmos.mhhe.com/
Gerd Keiser

Acknowledgments
In preparing this book and its previous editions, I am extremely grateful to the many people with whom
I had numerous discussions, who helped me in many different ways, and who supplied me with material
for these books. Special thanks go to Tri T. Ha, Naval Postgraduate School, who gave me the inspiration
and provided encouragement to write the ﬁrst edition. People from academia with whom I had many
beneﬁcial interactions and whose research publications were especially helpful include John Proakis,
Northeastern University and University of California San Diego; Selim Ünlü, Michael Ruane, and Malvin
Teich, Boston University; Shi-Shuenn Chen, San-Liang Lee, Cheng-Kuang Liu, and Shih-Hsiang Hsu,
National Taiwan University of Science and Technology (NTUST); Jean-Lien Chen Wu, St. John’s Uni-
versity (Taiwan); Perry Ping Shum, Nanyang Technological University; Hung-Chun Chang, Hen-Wei
Tsao, Jingshown Wu, and Chih-Chung Yang, National Taiwan University; Wood-Hi Cheng, National
Sun Yat-Sen University; Arthur Lowery, Monash University; Alan E. Willner, University of Southern
California; Daniel Blumenthal, University of California Santa Barbara; François Ladouceur, University
of New South Wales; Robert Minasian and Benjamin Eggleton, University of Sydney; Craig Armiento,
University of Massachusetts Lowell; Hui-Chi Chen, Fu-Jen Catholic University; Lian-Kuan Chen and
Hon Tsang, Chinese University of Hong Kong; Arthur Chiou and Fu-Jen Kao, National Yang Ming Uni-
versity; Bahaa Saleh and Guifang Li, University of Central Florida; El-Hang Lee, Inha University; Yun
Chung, KoreaAdvanced Institute of Science andTechnology; ChongqingWu, Beijing Jiaotong University;
Jintong Lin and Kun Xu, Beijing University of Posts and Telecommunications; Emily Jianzhong Hao,
Institute for Infocomm Research, Singapore; Heidi Abrahamse, University of Johannesburg; Richard
Penty and Ian White, Cambridge University; Sarah Dods, Royal Melbourne Institute of Technology
(RMIT); and Brian Culshaw, Strathclyde University. In addition, it was a great pleasure at NTUST to
interact with Chu-Lin Chang, Daniel Liang-Tang Chen, Olivia Devi Haobijam, Hsin-Yi Hsu, Shu-Min
Hsu, Kuo-Ming Huang, Ming-Je Huang, Yung-Jr Hung, Yu-Jhu Jhang, Chen-Yu Lee, Shu-Chuan Lin,
Zih-Rong Lin, Hua-Liang Lo, and Joni W. Simatupang.
Other people who assisted in various ways include William (Bill) Beck, Troy Bergstrom, Bertand
Destieux, Emmanuel Desurvire, Paul Fitzgerald, Doug Forster, Paul Fowler, Enrico Ghillino, André
Girard, Jim Hayes, Frank Jaffer, Jan Jakubczyk, Joy Changhong Jiang, Jack Kretovics, Hui-Ru Lin,
André Richter, Bruce Robertson, Rosmin Robertson, Dirk Seewald, Douglas Walsh, and Winston I. Way.
Special thanks go to the following people who helped provide photos: Simone Baldassarri, Evie Bennett,
Michael Kwok, Courtney McDaniel, Victoria McDonald, Robert E. Orr, Erika Peterssen, Jeff Sitlinger,
Randa Van Dyk, Brad Wackerlin, and Stephanie H. Webb.
I also am indebted to the reviewers who had very helpful suggestions for enhancing and clarifying
the material. They include Frank Barnes, University of Colorado; Nagwa Bekir, California State
University–Northridge; John A. Buck, Georgia Tech; Sang-Yeon Cho, New Mexico State University;
Ira Jacobs, Virginia Polytechnic Institute & State University; Raymond K. Kostuk, University ofArizona;
Peter LoPresti, University of Tulsa; Zhongqi Pan, University of Louisiana at Lafayetteville; Thomas K.
Plant, Oregon State University; Banmali Rawat, University of Nevada-Reno; Stephen Schultz, Brigham
Young University; and Huikai Xie, University of Florida. Particularly encouraging for doing the fourth

xxviii Acknowledgments
edition were all the positive comments on the previous editions received from users and adapters at
numerous academic institutions worldwide.
This edition especially benefited from the expert guidance of Lisa Bruflodt, Lorraine Buczek, Carrie
Burger, Peter Massar, and Raghu Srinivasan of McGraw-Hill, together with Deepti Narwat Agarwal of
Glyph International and Kay Mikel.As a final personal note, I am grateful to my wife Ching-yun and my
daughter Nishla for their patience and encouragement during the time I devoted to writing and revising
this book.
Gerd Keiser

CHAPTER 1
Overview of Optical Fiber
Communications
Ever since ancient times, people had a principal need to communicate with one another. This need created
interests in devising communication systems for sending messages from one distant place to another.
Optical communication methods were of special interest among the many systems that people tried to use.
One of the earliest known optical transmission links was a fire-signal method used by the Greeks in the
eighth century BC for sending alarms, calls for help, or announcements of certain events. Improvements
of these optical transmission systems were not pursued very actively due to technological limitations
at the time. For example, the speed of sending information over the communication link was limited
because the transmission rate depended on how fast the senders could move their hands, the optical signal
receiver was the error-prone human eye, line-of-sight transmission paths were required, and atmospheric
effects such as fog and rain made the transmission path unreliable. Thus it turned out to be faster, more
efficient, and more dependable to send messages by a courier over the road network.
Subsequently, no significant advances for optical communications appeared until the invention of the
laser in the early 1960s. Optical frequencies generated by such a coherent optical source are on the order
of 5 ¥ 1014
Hz, so the laser has a theoretical information capacity exceeding that of microwave systems
by a factor of 105
. With the potential of such wideband transmission capacities in mind, experiments using
atmospheric optical channels were carried out in the early 1960s. These experiments showed the feasibility
of modulating a coherent optical carrier wave at very high frequencies. However, the high cost of developing
and implementing such systems, together with the limitations imposed on the atmospheric optical channels
by rain, fog, snow, and dust, make such extremely high-speed links economically unattractive.
At the same time it was recognized that an optical fiber can provide a more reliable transmission
channel because it is not subject to adverse environmental conditions.1,2
Initially, the extremely large
losses of more than 1000 dB/km made optical fibers appear impractical. This changed in 1966 when Kao
and Hockman3
speculated that the high losses were a result of impurities in the fiber material, and that
the losses potentially could be reduced significantly in order to make optical fibers a viable transmission
medium. In 2009 Charles K. C. Kao was awarded the Nobel Prize in Physics for his pioneering insight
and his enthusiastic international follow-ups in promoting the further development of low-loss optical
fibers. These efforts led to the first ultrapure fiber being fabricated in 1970, only four years after his

2 Optical Fiber Communications
prediction. This breakthrough led to a series of technology developments related to optical fibers. These
events finally allowed practical lightwave communication systems to start being fielded worldwide in
1978. These systems operate in the near-infrared region of the electromagnetic spectrum (nominally 770
to 1675 nm) and use optical fibers as the transmission medium.
The goal of this book is to describe the various technologies, implementation methodologies, and
performance measurement techniques that make optical fiber communication systems possible. The
reader can find additional information on the theory of light propagation in fibers, the design of links and
networks, and the evolution of optical fibers, photonic devices, and optical fiber communication systems
in a variety of books and conference proceedings.4–23
This chapter gives an overview of fundamental communications concepts and illustrates how optical
fiber transmission systems operate. First, Sec. 1.1 gives the motivations behind the development of
optical fiber transmission systems. Next, Sec. 1.2 defines the different spectral bands that describe various
operational wavelength regions used in optical communications. Section 1.3 explains decibel notation
for expressing optical power levels. Section 1.4 gives the basic hierarchy for multiplexing digitized
information streams used on optical links, and Sec. 1.5 describes how wavelength division multiplexing
can boost the transmission capacity of an optical fiber significantly. Next, Sec. 1.6 introduces the functions
and implementation considerations of the key elements used in optical fiber systems.
An important aspect of realizing a smoothly interacting worldwide lightwave network is to have well-
established international standards for all aspects of components and networks. Section 1.7 introduces the
organizations involved with this standardization activity and lists the main classes of standards related
to optical communication components, system operations, and installation procedures. Finally, Sec. 1.8
gives an introduction to modeling and simulation tools that have been developed to aid in the design of
optical fibers, passive and active devices, links, and networks.
Chapters 2 through 10 describe the purpose and performance characteristics of the major elements in an
optical link. These elements include optical fibers, light sources, photodetectors, passive optical devices,
optical amplifiers, and active optoelectronic devices used in multiple-wavelength networks. Chapters 11
through 14 show how the elements are put together to form links and networks and explain measurement
methodologies used to evaluate the performance of lightwave components and links.
1.1 Motivations for Lightwave Communications
1.1.1 The Path to Optical Networks
Prior to about 1980 most communication technologies involved some type of electrical transmission
mechanism. The era of electrical communications started in 1837 with the invention of the telegraph by
Samuel F. B. Morse. The telegraph system used the Morse code, which represents letters and numbers
by a coded series of dots and dashes. The encoded symbols were conveyed by sending short and long
pulses of electricity over a copper wire at a rate of tens of pulses per second. More advanced telegraph
schemes, such as the Baudot system invented in 1874, enabled the information speeds to increase to about
120 bits per second (b/s) but required the use of skilled operators. Shortly thereafter in 1876 Alexander
Graham Bell developed a fundamentally different, easily useable device that could transmit the entire
voice signal in an analog form.
Both the telegraph and the analog voice signals were sent using a baseband transmission mode.
Baseband refers to the technology in which a signal is transmitted directly over a channel. For example,

Overview of Optical Fiber Communications 3
this method is used on standard twisted-pair wire links running from an analog telephone to the nearest
switching interface equipment. The same baseband method is used widely in optical communications;
that is, the optical output from a light source is turned on and off in response to the variations in voltage
levels of an information-bearing electrical signal.
In the ensuing years an increasingly larger portion of the electromagnetic spectrum was utilized to
develop and deploy progressively more sophisticated and reliable electrical communication systems with
larger capacities for conveying information from one place to another. The basic motivations behind each
new system application were (a) to improve the transmission fidelity so that fewer distortions or errors
occur in the received message, (b) to increase the data rate or capacity of a communication link so that
more information can be sent, or (c) to increase the transmission distance between in-line repeater or
amplification stations so that messages can be sent farther without the need to restore the signal amplitude
or fidelity periodically along its path. These activities led to the birth of a wide variety of communication
systems that are based on using high-capacity long-distance terrestrial and undersea copper-based wire
lines and wireless radio-frequency (RF), microwave, and satellite links.
In these developments the basic trend for advancing the link capacity was to use increasingly higher
channel frequencies. The reason for this trend is that a time-varying baseband information-bearing signal
may be transferred over a communication channel by superimposing it onto a sinusoidal electromagnetic
wave, which is known as the carrier wave or simply carrier.At the destination the baseband information
signal is removed from the carrier wave and processed as desired. Since the amount of information that
can be transmitted is directly related to the frequency range over which the carrier operates, increasing
the carrier frequency theoretically increases the available transmission bandwidth and, consequently,
provides a larger information capacity.24–28
For example, Fig. 1.1 shows the electromagnetic spectral
bands used for radio transmission. As the diverse radio technologies move from high frequency (HF) to
very high frequency (VHF) to ultra high frequency (UHF) bands with nominal carrier frequencies of 107
,
108
, and 109
Hz, respectively, increasingly higher information transmission speeds can be employed to
provide a higher link capacity. Thus the trend in electrical communication system developments was to
use progressively higher frequencies, which offer corresponding increases in bandwidth or information
capacity.
As Fig. 1.1 also shows, optical frequencies are several orders of magnitude higher than those used by
electrical communication systems. Thus the invention of the laser in the early 1960s aroused a curiosity
about the possibility of using the optical region of the electromagnetic spectrum for transmitting
information. Of particular interest is the near-infrared spectral band ranging from about 770 to
1675 nm, since this is a low-loss region in silica glass fibers.The technical breakthrough for optical fiber
communications started in 1970 when researchers at Corning demonstrated the feasibility of producing
a glass fiber having an optical power loss that was low enough for a practical transmission link.29
As research progressed, it became clear that many complex problems made it extremely difficult to
extend the carrier concept for achieving a super broadband optical communication link. Nevertheless,
the unique properties of optical fibers gave them a number of performance advantages compared to
copper wires, so that optical links operating in a simple on-off keyed baseband mode were attractive
applications.
The first installed optical fiber links appeared in the late 1970s and were used for transmitting telephony
signals at about 6 Mb/s over distances of around 10 km. As research and development progressed, the
sophistication and capabilities of these systems increased rapidly during the 1980s to create links carrying
aggregated data rates beyond terabits per second over distances of hundreds of kilometers without the
need to restore signal fidelity along the path length.

Starting in the 1990s there was a burgeoning demand on communication-network assets for bandwidth-
hungry services such as database queries, home shopping, high-deﬁnition interactive video, remote
education, telemedicine and e-health, high-resolution editing of home videos, blogging, and large-scale
high-capacity e-science and Grid computing.30–33
This demand was fueled by the rapid proliferation of
personal computers (PCs) coupled with a phenomenal increase in their storage capacity and processing
capabilities. Furthermore, the widespread availability and continuous expansion of the Internet, and an
extensive choice of remotely accessible programs and information databases, resulted in a dramatic rise
Fig. 1.1 The regions of the electromagnetic spectrum used for radio and optical ﬁber
communications. (Used with permission from A. B. Carlson, Communication
Systems, © 1986, McGraw-Hill Book Company.)
UHF TV
10–6 m
Ultraviolet
Visible
Infrared
Designation Transmission media Applications
Telephone
Data
Video
800 nm
2.55 m
μ
1015 Hz
1014 Hz
100 GHz
10 GHz
1 GHz
100 MHz
10 MHz
1 MHz
100 kHz
10 kHz
1 kHz
1 cm
10 cm
1 m
10 m
100 m
1 km
10 km
100 km
Millimeter
waves
Super high
frequency
(SHF)
Ultra high
frequency
(UHF)
Very high
frequency
(VHF)
High
frequency
(HF)
Medium
frequency
(MF)
Low
frequency
(LF)
Very low
frequency
(VLF)
Audio
Navigation
Satellite-to-satellite
Microwave relay
Earth-to-satellite
Radar
Mobile, Aeronautical
VHF TV and FM
Mobile radio
Business
Amateur radio
International
Citizen’s band
AM broad casting
Aeronautical
Submarine cable
Navigation
Transoceanic radio
Telephone
Telegraph
Frequency
Wavelength
Optical
fibers
Waveguide
Coaxial
cable
Wire
pairs
Microwave
radio
Shortwave
radio
Longwave
radio
Laser
beams

in PC usage. To handle the ever-increasing demand for high-bandwidth services ranging from home-
based PC users to large businesses and research organizations, telecommunication companies worldwide
greatly enhanced the capacity of fiber lines. This was accomplished by adding more independent signal-
carrying wavelengths on individual fibers and increasing the transmission speed of information being
carried by each wavelength.
1.1.2 Advantages of Optical Fibers
The advantages of optical fibers compared to copper wires include the following:
Long Distance Transmission Optical fibers have lower transmission losses compared to copper
wires. Consequently data can be sent over longer distances, thereby reducing the number of inter-
mediate repeaters needed to boost and restore signals in long spans. This reduction in equipment
and components decreases system cost and complexity.
Large Information Capacity Optical fibers have wider bandwidths than copper wires, so that
more information can be sent over a single physical line. This property decreases the number of
physical lines needed for sending a given amount of information.
Small Size and Low Weight The low weight and the small dimensions of fibers offer a distinct
advantage over heavy, bulky wire cables in crowded underground city ducts or in ceiling-mounted
cable trays. This feature also is of importance in aircraft, satellites, and ships where small, low-
weight cables are advantageous, and in tactical military applications where large amounts of cable
must be unreeled and retrieved rapidly.34
Immunity to Electrical Interference An especially important feature of an optical fiber relates
to the fact that it is a dielectric material, which means it does not conduct electricity. This makes
optical fibers immune to the electromagnetic interference effects seen in copper wires, such as
inductive pickup from other adjacent signal-carrying wires or coupling of electrical noise into the
line from any type of nearby equipment.
Enhanced Safety Optical fibers offer a high degree of operational safety because they do not
have the problems of ground loops, sparks, and potentially high voltages inherent in copper
lines. However, precautions with respect to laser light emissions need to be observed to prevent
possible eye damage.
Increased Signal Security An optical fiber offers a high degree of data security because the
optical signal is well-confined within the fiber and an opaque coating around the fiber absorbs any
signal emissions. This feature is in contrast to copper wires where electrical signals potentially
could be tapped off easily. Thus optical fibers are attractive in applications where information
security is important, such as financial, legal, government, and military systems.
1.2 Optical Spectral Bands
1.2.1 Electromagnetic Energy
All telecommunication systems use some form of electromagnetic energy to transmit signals. The
spectrum of electromagnetic (EM) radiation is shown in Fig. 1.2. Electromagnetic energy is a combination
of electrical and magnetic fields and includes power, radio waves, microwaves, infrared light, visible

light, ultraviolet light, X rays, and gamma rays. Each discipline takes up a portion (or band) of the
electromagnetic spectrum. The fundamental nature of all radiation within this spectrum is that it can be
viewed as electromagnetic waves that travel at the speed of light, which is about c = 3 ¥ 108
m/s in a
vacuum. Note that the speed of light s in a material is smaller by the refractive-index factor n than the
speed c in a vacuum, as described in Chapter 2. For example, n ª 1.45 for silica glass, so that the speed
of light in this material is about s = 2 ¥ 108
m/s.
The physical properties of the waves in different parts of the spectrum can be measured in several
interrelated ways. These are the length of one period of the wave, the energy contained in the wave, or
the oscillating frequency of the wave. Whereas electrical signal transmission tends to use frequency to
designate the signal operating bands, optical communication generally uses wavelength to designate
the spectral operating region and photon energy or optical power when discussing topics such as signal
strength or electro-optical component performance.
As can be seen from Fig. 1.2, there are three different ways to measure the physical properties
of a wave in various regions in the EM spectrum. These measurement units are related by some
simple equations. First of all, in a vacuum the speed of light c is equal to the wavelength l times the
frequency n, so that
c = ln (1.1)
where the frequency n is measured in cycles per second or hertz (Hz).
The relationship between the energy of a photon and its frequency (or wavelength) is determined by
the equation known as Planck’s Law
E = hn (1.2)
where the parameter h = 6.63 ¥ 10–34
J-s = 4.14 ¥ 10–15
eV-s is Planck’s constant. The unit J means joules
and the unit eV stands for electron volts. In terms of wavelength (measured in units of mm), the energy
Fig. 1.2 The spectrum of electromagnetic radiation
10–2
1010
108 1018
1016
1014
1012
103
Photon energy (eV)
HF/UHF/VHF Radio Microwaves
Infrared light
1.7 m to 1 mm
μ
Fiber optics
770 to 1675 nm
(~375 to 176 THz)
Ultraviolet
10–4 10–8
10–6
10–3 0.1 10
1
10–5
X rays
Visible light
400-700 nm
Frequency
(Hz)
Wavelength
(m)

in electron volts is given by
E( )
.
( )
eV
m
=
1 2406
λ μ
(1.3)
Figure 1.2 shows the optical spectrum ranges from about 5 nm in the ultraviolet region to 1 mm for far-
infrared radiation. In between these limits is the 400-to-700-nm visible band. Optical fiber communication
uses the near-infrared spectral band ranging from nominally 770 to 1675 nm.
The International Telecommunications Union (ITU) has designated six spectral bands for use in optical
fiber communications within the 1260-to-1675-nm region.35
These long-wavelength band designations
arose from the attenuation characteristics of optical fibers and the performance behavior of an erbium-
doped fiber amplifier (EDFA), as described in Chapters 3 and 10, respectively. Figure 1.3 shows and
Table 1.1 defines the regions, which are known by the letters O, E, S, C, L, and U.
The 770-to-910 nm band is used for shorter-wavelength multimode fiber systems. Thus this region
is designated as the short-wavelength or multimode fiber band. Later chapters describe the operational
performance characteristics and applications of optical fibers, electro-optic components, and other passive
optical devices for use in the short- and long-wavelength bands.
Example 1.1 Show that photon energies decrease
with increasing wavelength. Use wavelengths at 850,
1310, and 1550 nm.
Solution: FromEq.(1.3)wefindE(850nm)=1.46eV,
E(1310 nm) = 0.95 eV, and E(1550 nm) = 0.80 eV.
Table 1.1 Spectral band designations used in optical fiber communications
Name Designation Spectrum (nm) Origin of name
Original band O-band 1260 to 1360 Original (first) region used for single-mode fiber
links
Extended band E-band 1360 to 1460 Link use can extend into this region for fibers with low
water content
Short band S-band 1460 to 1530 Wavelengths are shorter than the C-band but higher
than the E-band
Conventional band C-band 1530 to 1565 Wavelength region used by a conventional EDFA
Long band L-band 1565 to 1625 Gain of an EDFA decreases steadily to 1 at 1625 nm in
this longer wavelength band
Ultra-long band U-band 1625 to 1675 Region beyond the response capability of an EDFA
Fig. 1.3 Designations of spectral bands used for optical fiber communications

1.2.2 Windows and Spectral Bands
Figure 1.4 shows the operating range of optical fiber systems and the characteristics of the four key
components of a link: the optical fiber, light sources, photodetectors, and optical amplifiers. Here the
dashed vertical lines indicate the centers of the three main traditional operating wavelength bands of
optical fiber systems, which are the short-wavelength region, the O-band, and the C-band. One of the
principal characteristics of an optical fiber is its attenuation as a function of wavelength, as shown at the
top in Fig. 1.4. Early applications in the late 1970s made exclusive use of the 770-to-910-nm wavelength
band where there was a low-loss window and GaAlAs optical sources and silicon photodetectors operating
at these wavelengths were available. Originally this region was referred to as the first window because
around 1000 nm there was a large attenuation spike due to absorption by water molecules. As a result of
this spike, early fibers exhibited a local minimum in the attenuation curve around 850 nm.
Fig. 1.4 Characteristics and operating ranges of the four key optical fiber link
components
10
1
700 900 1100 1300 1500 1700
1550 nm
1310 nm
850 nm
Optical fibers
Optical sources
GaAlAs InGaAsP
Optical amplifiers
Optical fiber amplifiers
PDFA EDFA
Photodetectors
1.0
0.5
Si
700 900 1100 1300 1500 1700
Wavelength (nm)
Ge
InGaAs
Responsivity
(A/W)
Attenuation
(dB/km)
First
window
Second
window
Third
window
TDFA
Raman amplification

By reducing the concentration of hydroxyl ions and metallic impurities in the fiber material, in the
1980s manufacturers could fabricate optical fibers with very low losses in the 1260-to-1675-nm region.
This spectral band is called the long-wavelength region. Because the glass still contained some water
molecules, a third-order absorption spike remained around 1400 nm. This spike defined two low-loss
windows, these being the second window centered at 1310 nm and the third window centered at 1550 nm.
These two windows now are called the O-band and C-band, respectively. The desire to use the low-
loss long-wavelength regions prompted the development of InGaAsP-based light sources and InGaAs
photodetectors that can operate at 1310 and 1550 nm. In addition, doping optical fibers with rare-earth
elements such as Pr, Th, and Er creates optical fiber amplifiers (called PDFA, TDFA, and EDFA devices,
respectively). These devices and the use of Raman amplification gave a further capacity boost to high-
capacity long-wavelength systems.
Special material-purification processes can eliminate almost all water molecules from the glass fiber
material, thereby dramatically reducing the water-attenuation peak around 1400 nm. This process opens
the E-band (1360-to-1460 nm) transmission region to provide around 100 nm more spectral bandwidth
in these specially fabricated fibers than in conventional single-mode fibers.
Systems operating at 1550 nm provide the lowest attenuation, but the signal dispersion in a standard
silica fiber is larger at 1550 nm than at 1310 nm. Fiber manufacturers overcame this limitation first
by creating dispersion-shifted fibers for single-wavelength operation and then by devising non-zero
dispersion-shifted fiber (NZDSF) for use with multiple-wavelength implementations. The latter fiber
type has led to the widespread use of multiple-wavelength S-band and C-band systems for high-capacity,
long-span terrestrial and undersea transmission links. These links routinely carry traffic at 10 Gb/s over
nominally 90-km distances between amplifiers or repeaters. By 2005 links operating at 40 Gb/s were being
installed and field trials of 160-Gb/s long-distance transmission systems were tested successfully.36–39
1.3 Decibel Units
As described in all of the following chapters of this book, a critical consideration when designing and
implementing an optical fiber link is to establish, measure, and/or interrelate the optical signal levels at
each of the elements of a transmission link. Thus it is necessary to know parameter values such as the
optical output power from a light source, the power level needed at the receiver to properly detect a signal,
and the amount of optical power lost at the constituent elements of the transmission link.
Reduction or attenuation of signal strength arises from various loss mechanisms in a transmission
medium. For example, electric power is lost through heat generation as an electric signal flows along a
wire, and optical power is attenuated through scattering and absorption processes in a glass or plastic fiber
or in an atmospheric channel. To compensate for these energy losses, amplifiers are used periodically
along a channel path to boost the signal level, as shown in Fig. 1.5.
Astandard and convenient method for measuring attenuation through a link or a device is to reference
the output signal level to the input level. For guided media such as an optical fiber, the signal strength
normally decays exponentially. Thus for convenience one can designate it in terms of a logarithmic power
ratio measured in decibels (dB). The dB unit is defined by
Power ratio in dB = 10 log
P
P
2
1
(1.4)
where P1 and P2 are the electrical or optical power levels of a signal at points 1 and 2 in Fig. 1.6, and
log is the base-10 logarithm. The logarithmic nature of the decibel allows a large ratio to be expressed

in a fairly simple manner. Power levels differing by many
orders of magnitude can be compared easily when they are
in decibel form. Another attractive feature of the decibel
is that to measure the changes in the strength of a signal,
one merely adds or subtracts the decibel numbers between
two different points.
Table 1.2 shows some sample values of power loss
given in decibels and the percent of power remaining
after this loss. These types of numbers are important when
considering factors such as the effects of tapping off a
small part of an optical signal for monitoring purposes, for
examining the power loss through some optical element,
or when calculating the signal attenuation in a specific
length of optical fiber.
Table 1.2 Representative values
of decibel power loss and the
remaining percentages
Power loss Percent of
(in dB) power left
0.1 98
0.5 89
1 79
2 63
3 50
6 25
10 10
20 1
Fig. 1.6 Example of pulse attenuation in a link. P1 and P2 are the power levels of a signal
at points 1 and 2
P1
P P
2 1
= 0.5
Transmission line
Point 1 Point 2
Fig. 1.5 Periodically placed amplifiers compensate for energy losses along a link

Since the decibel is used to refer to ratios or relative
units, it gives no indication of the absolute power level.
However, a derived unit can be used for this purpose.
Such a unit that is particularly common in optical fiber
communications is the dBm (simply pronounced dee bee
em). This unit expresses the power level P as a logarithmic
ratio of P referred to 1 mW. In this case, the power in dBm
is an absolute value defined by
Power level (in dBm) 10 log
in mW)
1 mW
=
P( (1.5)
An important rule-of-thumb relationship to remember
for optical fiber communications is 0 dBm = 1 mW.
Therefore, positive values of dBm are greater than 1 mW
and negative values are less than 1 mW.
Table 1.3 lists some examples of optical power levels
and their dBm equivalents.
Table 1.3 Examples of optical
power levels and their dBm
equivalents
Power dBm equivalent
200 mW 23
100 mW 20
10 mW 10
1 mW 0
100 mW –10
10 mW –20
1 mW –30
100 nW –40
10 nW –50
1 nW –60
100 pW –70
10 pW –80
1 pW –90
Example 1.3 Consider the transmission path from
point 1 to point 4 shown in Fig. 1.7. Here the signal is
attenuated by 9 dB between points 1 and 2. After getting
a 14-dB boost from an amplifier at point 3, it is again
attenuated by 3 dB between points 3 and 4. Relative to
point 1, the signal level in dB at point 4 is
dB level at point 4 = (loss in line 1) + (amplifier gain)
+ (loss in line 2)
= (–9 dB) + (14 dB) + (–3 dB) = + 2 dB
Thus the signal has a 2-dB (a factor of 100.2
= 1.58)
gain in power in going from point 1 to point 4.
Example 1.2 Assume that after traveling a certain
distance in some transmission medium, the power of a
signal is reduced to half, that is, P2 = 0.5 P1 in Fig. 1.6.
At this point, using Eq. (1.4) the attenuation or loss of
power is
10 log
P
P
2
1
= 10 log
0 5 1
1
. P
P
= 10 log 0.5 = 10(–0.3) = –3 dB
Thus, –3 dB (or a 3-dB attenuation or loss) means
that the signal has lost half its power. If an amplifier is
inserted into the link at this point to boost the signal back
to its original level, then that amplifier has a 3-dB gain.
If the amplifier has a 6-dB gain, then it boosts the signal
power level to twice the original value.
Fig. 1.7 Example of signal attenuation and amplification in a transmission path
Amplifier
Point 3
Point 2
Point 1
Transmission line 1
Point 4
Transmission line 2
– 3 dB
+ 14 dB
+ 2 dB
– 9 dB

1.4 Network Information Rates
To handle the continuously rising demand for high-bandwidth services from users ranging from
individuals to large businesses and research organizations, telecommunication companies worldwide are
implementing increasingly sophisticated digital multiplexing techniques that allow a larger number of
independent information streams to share the same physical transmission channel simultaneously. This
section describes some common digital signal multiplexing techniques.
1.4.1 Telecom Signal Multiplexing
Table 1.4 gives examples of information rates for some typical telecom services. To send these
services from one user to another, network providers combine the signals from many different users
and send the aggregate signal over a single transmission line. This scheme is known as time-division-
multiplexing (TDM) wherein N independent information streams, each running at a data rate of R b/s,
are interleaved electrically into a single information stream operating at a higher rate of N ¥ R b/s.
To get a detailed perspective of this methodology, let us look at the multiplexing schemes used in
telecommunications.
Early applications of fiber optic transmission links were mainly for large capacity telephone lines.
These digital links consisted of time-division-multiplexed 64-kb/s voice channels. The multiplexing was
developed in the 1960s and is based on what is known as the plesiochronous digital hierarchy (PDH).
Figure 1.8 shows the digital transmission hierarchy used in the North American telephone network.
Table 1.4 Examples of information rates for some typical services
Type of service Data rate
Video on demand/interactive TV 1.5 to 6 Mb/s
Video games 1 to 2 Mb/s
Remote education 1.5 to 3 Mb/s
Electronic shopping 1.5 to 6 Mb/s
Data transfer or telecommuting 1 to 3 Mb/s
Video conferencing 0.384 to 2 Mb/s
Voice (single telephone channel) 33.6 to 56 kb/s
Example 1.4 Consider three different light
sources having the following optical output powers:
50 mW, 1 mW, and 50 mW. What are the power levels
in dBm units?
Solution: Using Eq. (1.5) to express the light
levels in dBm units, we find that the output powers
of these sources are –13 dBm, 0 dBm, and +17 dBm,
respectively.
Example 1.5 Consider a product data sheet for
a photodetector that states that an optical power level
of –32 dBm is needed at the photodetector to satisfy a
specific performance requirement. What is the power
level in nW (nanowatt) units?
Solution: From Eq. (1.5) we find that a –32-dBm
level corresponds to a power in nW of
P = 10–32/10
mW = 0.631 mW = 631 nW

The fundamental building block is a 1.544-Mb/s transmission rate known as a DS1 rate, where DS stands
for digital system. It is formed by time-division-multiplexing twenty-four voice channels, each digitized
at a 64-kb/s rate (which is referred to as DS0). Framing bits, which indicate where an information unit
starts and ends, are added along with these voice channels to yield the 1.544-Mb/s bit stream. Framing
and other control bits that may get added to an information unit in a digital stream are called overhead
bits. At any multiplexing level a signal at the designated input rate is combined with other input signals
at the same rate.
DSx versus Tx In describing telephone network data rates, one sees terms such as T1, T3, and so
on. Often the terms Tx and DSx (e.g., T1 and DS1 or T3 and DS3) are used interchangeably. However,
there is a subtle difference in their meaning. Designations such as DS1, DS2, and DS3 refer to a service
type; for example, a user who wants to send information at a 1.544 Mb/s rate would subscribe to a DS1
service.Abbreviations such as T1, T2, and T3 refer to the data rate the transmission-line technology uses
to deliver that service over a physical link. For example, the DS1 service is transported over a physical
wire or optical fiber using electrical or optical pulses sent at a T1 = 1.544 Mb/s rate.
The TDM scheme is not restricted to multiplexing voice signals. For example, at the DS1 level, any
64-kb/s digital signal of the appropriate format could be transmitted as one of the 24 input channels shown
in Fig. 1.8. As noted there and in Table 1.5, the main multiplexed rates for North American applications
are designated as DS1 (1.544 Mb/s), DS2 (6.312 Mb/s), and DS3 (44.736 Mb/s). European and Japanese
networks define similar hierarchies using different bit-rate levels as shown in Table 1.5. In Europe the
multiplexing hierarchy is labeled E1, E2, E3, and so on.
1.4.2 SONET/SDH Multiplexing Hierarchy
With the advent of high-capacity fiber optic transmission lines in the 1980s, service providers established
a standard signal format called synchronous optical network (SONET) in NorthAmerica and synchronous
Fig. 1.8 Digital transmission hierarchy used in the North American telephone network
T4
multiplexer
274.176 Mb/s
1
6
T3
multiplexer
44.736 Mb/s
1
7
Six
44.736-Mb/s
inputs
T2
multiplexer
6.312 Mb/s
1
4
Seven
6.312-Mb/s
inputs
T1
multiplexer
1.544 Mb/s
1
24
Seven
1.544-Mb/s
inputs
64-kb/s
inputs
Example 1.6 As can be seen from Fig. 1.8, at
each multiplexing level some overhead bits are added for
synchronization purposes. What is the overhead for T1?
Solution: At T1 the overhead is
1544 kb/s – 24 ¥ 64 kb/s = 8 kb/s

digital hierarchy (SDH) in other parts of the world.40–42
These standards define a synchronous frame
structure for sending multiplexed digital traffic over optical fiber trunk lines. The basic building block and
first level of the SONET signal hierarchy is called the Synchronous Transport Signal - Level 1 (STS-1),
which has a bit rate of 51.84 Mb/s. Higher-rate SONET signals are obtained by byte-interleaving N of
these STS-1 frames, which then are scrambled and converted to an Optical Carrier - Level N (OC-N)
signal. Thus the OC-N signal will have a line rate exactly N times that of an OC-1 signal. For SDH systems
the fundamental building block is the 155.52-Mb/s Synchronous Transport Module - Level 1 (STM-1).
Again, higher-rate information streams are generated by synchronously multiplexing N different STM-1
signals to form the STM-N signal. Table 1.6 shows commonly used SDH and SONET signal levels, the
line rate, and the popular numerical name for that rate.
Table 1.6 Common SDH and SONET line rates and their popular numerical name
SONET level Electrical level SDH level Line rate (Mb/s) Popular rate name
OC-1 STS-1 — 51.84 —
OC-3 STS-3 STM-1 155.52 155 Mb/s
OC-12 STS-12 STM-4 622.08 622 Mb/s
OC-48 STS-48 STM-16 2488.32 2.5 Gb/s
OC-192 STS-192 STM-64 9953.28 10 Gb/s
OC-768 STS-768 STM-256 39813.12 40 Gb/s
Table 1.5 Digital multiplexing levels used in North America, Europe, and Japan
Digital multiplexing Number of 64-kb/s Bit rate (Mb/s)
level channels
North America Europe Japan
DS0 1 0.064 0.064 0.064
DS1 24 1.544 1.544
30 2.048
48 3.152 3.152
DS2 96 6.312 6.312
120 8.448
DS3 480 34.368 32.064
672 44.736
1344 91.053
1440 97.728
DS4 1920 139.264
4032 274.176
5760 397.200

1.5 WDM Concepts
The use of wavelength division multiplexing
(WDM) offers a further boost in fiber
transmission capacity. The basis of WDM is
to use multiple sources operating at slightly
different wavelengths to transmit several
independent information streams simultan-
eously over the same fiber. Figure 1.9 shows
the basic WDM concept. Here N independent
optically formatted information streams, each
transmitted at a different wavelength, are
combined by means of an optical multiplexer
and sent over the same fiber. Note that each of these streams could be at a different data rate. Each
information stream maintains its individual data rate after being multiplexed with the other traffic streams,
and still operates at its unique wavelength. Conceptually, the WDM scheme is the same as frequency
division multiplexing (FDM) used in microwave radio and satellite systems.
Although researchers started looking at WDM techniques in the 1970s, during the ensuing years
it generally turned out to be easier to transmit only a single wavelength on a fiber using high-speed
electronic and optical devices, than to invoke the greater system complexity called for in WDM. However,
a dramatic surge in WDM popularity started in the early 1990s owing to several factors. These include
new fiber types that provide better performance of multiple-wavelength operation at 1550 nm, advances
in producing WDM devices that can separate closely spaced wavelengths, and the development of optical
amplifiers that can boost C-band optical signal levels completely in the optical domain.
1.6 Key Elements of Optical Fiber Systems
Similar to electrical communication systems, the basic function of an optical fiber link is to transport
a signal from communication equipment (e.g., a computer, telephone, or video device) at one location
to corresponding equipment at another location with a high degree of reliability and accuracy.
Figure 1.10 shows the main constituents of an optical fiber communications link. The key sections are
Fig. 1.9 Basic concept of wavelength division
multiplexing
λN
Individual
fiber lines
Optical
multiplexer
Single
fiber line
λ1
λ2 λ1, λ2, ..., λN
Fig. 1.10 Main constituents of an optical fiber communications link
Optical
transmitter
Optical
receiver
Cabled optical fibers
Optical amplifier
Information
sources
Video
camera
Optical connecters
Passive or active optical devices
(optical filters, couplers, switches)
Information
recipients

a transmitter consisting of a light source and its associated drive circuitry, a cable offering mechanical
and environmental protection to the optical fibers contained inside, and a receiver consisting of a
photodetector plus amplification and signal-restoring circuitry. Additional components include optical
amplifiers, connectors, splices, couplers, regenerators (for restoring the signal-shape characteristics), and
other passive components and active photonic devices.
Undersea regenerator
or optical amplifier
Undersea
optical cable
Regenerator
Directly buried
fiber cable
Fiber cables in
underground
ducts
Fiber cables
within
buildings
Manhole
Aerial-mounted cable
Fig. 1.11 Optical fiber cables can be installed on poles, in ducts, and underwater, or they
can be buried directly in the ground.

The cabled fiber is one of the most important elements in an optical fiber link as is described in
Chapters 2 and 3. In addition to protecting the glass fibers during installation and service, the cable may
contain copper wires for powering optical amplifiers or signal regenerators, which are needed periodically
in long-distance links for amplifying and reshaping the signal. A variety of fiber types with different
performance characteristics exist for a wide range of applications. To protect the glass fibers during
installation and service, there are many different cable configurations depending on whether the cable is
to be installed inside a building, underground in ducts or through direct-burial methods, outside on poles,
or under water. Very low-loss optical connectors and splices are needed in all categories of optical fiber
networks for joining cables and for attaching one fiber to another.
Analogous to copper cables, the installation of optical fiber cables can be either aerial, in ducts,
undersea, or buried directly in the ground, as illustrated in Fig. 1.11. As described in Chapter 2, the
cable structure will vary greatly depending on the specific application and the environment in which it
will be installed. Owing to installation and/or manufacturing limitations, individual cable lengths for in-
building or terrestrial applications will range from several hundred meters to several kilometers. Practical
considerations such as reel size and cable weight determine the actual length of a single cable section.
The shorter segments tend to be used when the cables are pulled through ducts. Longer lengths are used
in aerial, direct-burial, or underwater applications.
Workers can install optical fiber cables by pulling or blowing them through ducts (both indoor and
outdoor) or other spaces, laying them in a trench outside, plowing them directly into the ground, suspending
them on poles, or laying or plowing them underwater.Although each method has its own special handling
procedures, they all need to adhere to a common set of precautions. These include avoiding sharp bends
of the cable, minimizing stresses on the installed cable, periodically allowing extra cable slack along
the cable route for unexpected repairs, and avoiding excessive pulling or hard yanks on the cable. For
direct-burial installations a fiber optic cable can be plowed directly underground or placed in a trench
that is filled in later. Figure 1.12 illustrates a plowing operation that may be carried out in nonurban
areas. The cables are mounted on large reels on the plowing vehicle and are fed directly into the ground
by means of the plow mechanism.
Fig. 1.12 Plowing operation for direct burial of optical fiber cables. (Photo © VERMEER
CORPORATION. All Rights Reserved; www.vermeer.com.)

Transoceanic cable lengths can be many
thousandsofkilometerslongandincludeperi-
odically spaced (on the order of 80–120 km)
optical repeaters to boost the signal level. The
cables are assembled in onshore factories and
thenareloadedintospecialcable-layingships,
as illustrated in Fig. 1.13. Splicing together
individual cable sections forms continuous
transmission lines for these long-distance
links.
Once the cable is installed, a transmitter
can be used to launch a light signal into the
fiber.Thetransmitterconsistsofalightsource
that is dimensionally compatible with the
fiber core and associated electronic control
and modulation circuitry. Semiconductor
light-emitting diodes (LEDs) and laser
diodes are suitable for this purpose. For
these devices the light output can be
modulated rapidly by simply varying the
input current at the desired transmission
rate, thereby producing an optical signal.
The electric input signals to the transmitter
circuitry for driving the optical source can
be either of an analog or digital form. The
functions of the associated transmitter electronics are to set and stabilize the source operating point and
output power level. For high-rate systems (usually greater than about 2.5 Gb/s), direct modulation of the
source can lead to unacceptable optical signal distortion. In this case, an external modulator is used to
vary the amplitude of a continuous light output from a laser diode source. In the 770-to-910-nm region
the light sources are generally alloys of GaAlAs. At longer wavelengths (1260 to 1675 nm) an InGaAsP
alloy is the principal optical source material.
After an optical signal is launched into a fiber, it will become progressively attenuated and distorted
with increasing distance because of scattering, absorption, and dispersion mechanisms in the glass material.
At the destination of an optical fiber transmission line, there is a receiving device that interprets the
information contained in the optical signal. Inside the receiver is a photodiode that detects the weakened
and distorted optical signal emerging from the end of an optical fiber and converts it to an electrical signal
(referred to as a photocurrent). The receiver also contains electronic amplification devices and circuitry
to restore signal fidelity. Silicon photodiodes are used in the 770-to-910-nm region. The primary material
in the 1260-to-1675-nm region is an InGaAs alloy.
The design of an optical receiver involves the requirement to correctly interpret the content of the
weakened and degraded signal received by the photodetector. The principal figure of merit for a receiver is
the minimum optical power necessary at the desired data rate to attain either a given error probability for
digital systems or a specified signal-to-noise ratio for an analog system. The ability of a receiver to achieve
a certain performance level depends on the photodetector type, the effects of noise in the system, and the
characteristics of the successive amplification stages in the receiver.
Fig. 1.13 Ship used to lay optical fiber cables across
a sea or an ocean. (Photo courtesy of TE
SubCom: www.SubCom.com.)

Included in any optical fiber link are various passive optical devices that assist in controlling and
guiding the light signals. Passive devices are optical components that require no electronic control for
their operation.Among these are optical filters that select only a narrow spectrum of desired light, optical
splitters that divide the power in an optical signal into a number of different branches, optical multiplexers
that combine signals from two or more distinct wavelengths onto the same fiber (or that separate the
wavelengths at the receiving end) in multiple-wavelength optical fiber networks, and couplers used to
tap off a certain percentage of light, usually for performance monitoring purposes.
In addition, modern sophisticated optical fiber networks contain a wide range of active optical
components, which require an electronic control for their operation. These include light signal modulators,
tunable (wavelength-selectable) optical filters, reconfigurable elements for adding and dropping
wavelengths at intermediate nodes, variable optical attenuators, and optical switches.
After an optical signal has traveled a certain distance along a fiber, it becomes greatly weakened due
to power loss along the fiber. Therefore, when setting up an optical link, engineers formulate a power
budget and add amplifiers or repeaters when the path loss exceeds the available power margin. The
periodically placed amplifiers merely give the optical signal a power boost, whereas a repeater also will
attempt to restore the signal to its original shape. Prior to 1990, only repeaters were available for signal
amplification. For an incoming optical signal, a repeater performs photon-to-electron conversion, electrical
amplification, retiming, pulse shaping, and then electron-to-photon conversion. This process can be fairly
complex for high-speed multiwavelength systems. Thus researchers expended a great deal of effort to
develop all-optical amplifiers, which boost the light power level completely in the optical domain. Optical
amplification mechanisms for WDM links include the use of devices based on rare-earth-doped lengths
of fiber and distributed amplification by means of a stimulated Raman scattering effect.
The installation and operation of an optical fiber communication system require measurement
techniques for verifying that the specified performance characteristics of the constituent components are
satisfied. In addition to measuring optical fiber parameters, system engineers are interested in knowing
the characteristics of passive splitters, connectors, and couplers, and electro-optic components, such as
sources, photodetectors, and optical amplifiers. Furthermore, when a link is being installed and tested,
operational parameters that should be measured include bit error rate, timing jitter, and signal-to-noise
ratio as indicated by the eye pattern. During actual operation, measurements are needed for maintenance
and monitoring functions to determine factors such as fault locations in fibers and the status of remotely
located optical amplifiers.
1.7 Standards for Optical Fiber Communications
To allow components and equipment from different vendors to interface with one another, numerous
international standards have been developed.43–45
The three basic classes for fiber optics are primary
standards, component testing standards, and system standards.
Primary standards refer to measuring and characterizing fundamental physical parameters such
as attenuation, bandwidth, operational characteristics of fibers, and optical power levels and spectral
widths. In the USA the main organization involved in primary standards is the National Institute of
Standards and Technology (NIST). This organization carries out fiber optic and laser standardization
work, and it sponsors an annual conference on optical fiber measurements. Other national organizations
include the National Physical Laboratory (NPL) in the United Kingdom and the Physikalisch-Technische
Bundesanstalt (PTB) in Germany.

Component testing standards define tests for fiber-optic component performance and
establish equipment-calibration procedures. Several different organizations are involved in formulating
testing standards, some very active ones being the Telecommunications Industry Association (TIA)
in association with the Electronics Industries Alliance (EIA), the Telecommunication Sector of the
International Telecommunication Union (ITU-T), and the International Electrotechnical Commission
(IEC). The TIA has a list of over 120 fiber optic test standards and specifications under the general
designation TIA/EIA-455-XX-YY, where XX refers to a specific measurement technique and YY refers
to the publication year. These standards are also called Fiber Optic Test Procedures (FOTP), so that
TIA/EIA-455-XX becomes FOTP-XX. These include a wide variety of recommended methods for testing
the response of fibers, cables, passive devices, and electro-optic components to environmental factors
and operational conditions. For example, TIA/EIA-455-60-1997, or FOTP-60, is a method published in
1997 for measuring fiber or cable length.
System standards refer to measurement methods for links and networks. The major organizations
are theAmerican National Standards Institute (ANSI), the Institute for Electrical and Electronic Engineers
(IEEE), the ITU-T, and Telcordia Technologies. Of particular interest for fiber optics system are test
standards and recommendations from the ITU-T. Within the G series (in the number range G.650 and
higher) the recommendations relate to fiber cables, optical amplifiers, wavelength multiplexing, optical
transport networks (OTN), system reliability and availability, and management and control for passive
optical networks (PON). The L and O series of the ITU-T address methods and equipment for the
construction, installation, maintenance support, monitoring, and testing of cable and other elements in
the optical fiber outside plant, that is, the fielded cable system. Telcordia Technologies provides a wide
range of generic requirements for telecommunication network components and systems. For example, the
GR-3120 Generic Requirements for Hardened Fiber Optic Connectors document describes the necessary
specifications for optical connectors that are environmentally hardened and can be mated in the field.
1.8 Modeling and Simulation Tools
Computer-based simulation and modeling tools that integrate component, link, and network functions
can make the design process of complex optical links and networks more efficient, less expensive, and
faster.46–51
The rapid proliferation and increase in capabilities of personal computers led to the development
of many sophisticated simulation programs for these machines for predicting photonic component, link,
and network performance behavior. These software tools are based on well-established numerical models
and can simulate factors such as connector losses due to geometric or position mismatches of fibers,
efficiencies of coupling optical power from light sources into fibers, behaviors of passive and active
optical components, and the performance of complex optical networks. They also can model many types
of passive and active devices, such as waveguide couplers, optical filters, waveguide grating arrays, and
optical sources, to a high degree of sophistication.
1.8.1 Simulation Tool Characteristics
Computer-aided design (CAD) tools can offer a powerful method to assist in analyzing the design of an
optical component, circuit, or network before costly prototypes are built. Important points to consider,
however, are the approximations and modeling assumptions made in the software design. Since most
telecommunication systems are designed with several decibels of safety margin, approximations for
calculating operating behavior that are reasonably accurate are not only acceptable but in general necessary
to allow tractable computation times.

The theoretical models used in computer simulations nominally include the following characteristics:
∑ Enough detail so that all factors that could influence the performance of the component, circuit,
or network can be appropriately evaluated.
∑ A common set of parameters so that simulated devices can be interconnected with each other to
form circuits or networks.
∑ Interfaces that pass sufficient information between the constituent components so that all possible
interactions are identified.
∑ Computational efficiency that allows a trade-off between accuracy and speed, so that quick estimates
of system performance can be made in the early stages of a design.
∑ The capability to simulate devices over the desired spectral bandwidth.
∑ The ability to simulate factors such as nonlinear effects, crosstalk between optical channels,
distortion in lasers, and dispersion in optical fibers.
To enable a user to visualize and simulate a system quickly, the simulation programs normally have
the following features:
∑ The ability to create a system schematic based on a library of graphical icons and a graphical user
interface (GUI). The icons represent various system components (such as optical fibers, filters,
amplifiers) and instrumentation (e.g., data sources, power meters, and spectrum analyzers).
∑ The ability for the user to interact with the program during a simulation. For example, the user
may want to modify a parameter or some operating condition in order to evaluate its effect. This
is especially important in the early stages of a design when the operating range of interest is being
established.
∑ A wide range of statistical-analysis, signal-processing, and display tools.
∑ Common display formats, including time waveforms, electrical and optical spectra, eye diagrams,
and error-rate curves.
1.8.2 Graphical Programming
Commercially available simulation tools for lightwave applications are based on well-established
graphical-programming languages. In these languages, system components (e.g., lasers, modulators,
optical amplifiers, optical fibers), measurement instruments, and plotting tools are represented by a
module library of programmed icons that have bidirectional optical and electrical interfaces. For example,
as Fig. 1.14 illustrates, the left-hand side of a typical modeling tool window lists classes of items such
as lasers, optical receivers, optical fibers, and measurement equipment. Using a mouse to click on a
specific class will open a window with a selection of icons for that class. Associated with each icon is a
menu window where the user specifies the values of the operational parameters of the component and its
interface characteristics. In addition to using preprogrammed modules, users can create their own custom
devices with either the underlying software code or the graphical-programming language.
Using such a set of graphical icons, one can put together a simulation of a complex component, a
simple link, or a sophisticated multichannel transmission path in several minutes. One simply selects the
icons representing the desired components and measurement instruments, and connects them together
with a wiring tool to create a model of the optical transmission system. When the design is completed, the
diagram compiles rapidly and the simulation can be run using control buttons on a tool bar. Figure 1.14
also shows an example of a simple link consisting of a 1-mW optical transmitter, a 20-km fiber, an optical
receiver, and an optical spectrum analyzer (OSA). Figure 1.15 illustrates the performance simulation
result of this link in the form of an eye diagram for the bit error rate (see Chapter 14) at a 10 Gb/s data
transmission rate.

Fig. 1.14 A screen view showing the tools used to create a simulation schematic using a
modeling program from VPIphotonics.
Fig. 1.15 A screen view showing an eye-diagram result from the simulation schematic
shown in Fig. 1.14.

Once the icons have been selected and connected together, the complex and challenging part starts for
the user. This involves choosing realistic ranges of the parameters of the electrical and optical components
and submodules. It is important that the parameter values make sense in an actual application. In some
cases this may entail examining the specification sheets of vendors.
1.8.3 Example Programs for Student Use
Several commercial vendors offer various suites of software-based modeling-tool modules for optical
fiber communication systems.52–54
These design and planning tools are intended for use across all levels
of lightwave network analyses, performance evaluations, and technology comparisons ranging from
passive and active components and modules, to complex transmission links, to entire optical networks.
Familiar measurement instruments are built into the software to offer a wide range of settable options
when displaying data from multiple simulation runs and multidimensional sweeps across a parameter
range. Signal-processing modules allow data to be manipulated to mimic any laboratory setup.
Such tools are in use by component and system manufacturers, system integrators, network operators,
and access service providers for functions such as capacity planning, comparative assessments of various
technologies, optimization of transport and service networks, syntheses and analyses of WDM system
and link designs, and component designs. In addition, many universities are using these simulation tools
for both research and teaching purposes.
Abbreviated versions of several simulation modules may be downloaded for noncommercial educational
use from the websites of the tool vendors. These simplified versions contain predefined component and
link configurations that allow interactive concept demonstrations. Among the numerous demonstration
setups are optical amplifier structures, simple single-wavelength links, and WDM links. The configurations
are fixed, but the reader has the ability to interactively change the operational parameter values of
components such as optical fibers, light sources, optical filters, and optical amplifiers. As part of the
results that are possible, images of what would appear on the display screen of a standard instrument,
such as a spectrum analyzer or an oscilloscope, enable the user to see the effects on link performance
when various component values change.
The book website (http://www.mhhe.com/keiserOFC) describes current offerings available on vendor
websites of the demonstration modules that the reader may download and learn from.55
Problems
1.1 (a) What are the energies in electron volts (eV)
of light at wavelengths 850, 1310, 1490,
and 1550 nm?
(b) Consider a 1-ns pulse with a 100-nW
amplitude at each of these wavelengths.
How many photons are in such a pulse at
each wavelength?
1.2 A WDM optical transmission system is
designed so that each channel has a spectral
width of 0.8 nm. How many wavelength
channels can be used in the C-band?
1.3 Three sine waves have the following periods:
25 ms, 250 ns, 125 ps. What are their
frequencies?
1.4 Asine wave is offset 1/6 of a cycle with respect
to time zero. What is its phase in degrees and
in radians?
1.5 Consider two signals that have the same
frequency.Whentheamplitudeofthefirstsignal
is at its maximum, the amplitude of the second
signalisathalfitsmaximumfromthezerolevel.
What is the phase shift between the two signals?
1.6 What is the duration of a bit for each of the
following three signals which have bit rates of
64 kb/s, 5 Mb/s, and 10 Gb/s?
1.7 (a) Convert the following absolute power gains
to decibel power gains: 10–3
, 0.3, 1, 4, 10,
100, 500, 2n
.

Gerd Keiser - Optical Fiber Communications-McGraw-Hill Education (2010).pdf

Gerd Keiser - Optical Fiber Communications-McGraw-Hill Education (2010).pdf

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Gerd Keiser - Optical Fiber Communications-McGraw-Hill Education (2010).pdf